It is well known that PMD may be measured by injecting wideband linearly polarized light into the waveguide, passing the light leaving the waveguide through a linear polarizer and then sending it to a scanning interferometer, recombining the light from the two arms of the interferometer to produce interferograms, converting the recombined light intensity, I, into a corresponding electrical signal and processing the electrical signal to extract the PMD from a curve, I(τ), of intensity I against the path delay difference τ between the two arms of the interferometer (referred to hereinafter as delay τ).
As explained in U.S. Pat. No. 5,712,704 (Martin et al.), the curve I(τ) exhibits a high central peak, with smaller fluctuations that can be seen on either side. (Similar fluctuations at the centre are masked by the central peak.) The central peak is representative of the light spectrum at the output of the waveguide (before the linear polarizer) while the fluctuations represent the PMD. According to Martin et al., this peak “hinders the processing of the detected signal, thereby constituting an obstacle to the accurate measurement of the PMD”.
Martin et al. discussed a previously-disclosed technique which removed the central peak by passing the light by way of a first polarizer at 45° to an interferometer having a polarizer at 0° in one arm and a polarizer at 90° in its other arm, and passing the recombined light through an analyzer (analysis polarizer) at 0°. Having identified complex construction and the use of the analyzer as unsatisfactory, Martin et al. sought to provide a simpler way of removing the central peak. In particular, Martin et al. provided at least one birefringent element having two independent modes of polarization in at least one of the arms of the interferometer, the algebraic sum of the elementary phase shifts in the two arms being equal to a relative phase shift and the algebraic difference between the two relative phase shifts having a value of π. According to Martin et al., this allows the curve of intensity I against delay τ to be obtained without the “parasitic” central peak.
Neither of these approaches is entirely satisfactory, however, because they are predicated upon removal of the “parasitic” central peak on the grounds that it detracts from the measurement of PMD, so that simple removal of the central peak should significantly improve measurement accuracy. This is only true in part. In fact, removing the central peak means that useful information is discarded, as a result of which very small PMDs, tending towards zero, cannot be measured accurately. In practice, improvement of measurement accuracy is marginal.